1. Field of the Invention
This invention relates to new prepolymers containing free isocyanate groups obtained by the reaction of excess quantities of polyisocyanates containing isocyanurate groups with polyester polyols, to the prepolymers obtainable by this process and to their use as binders in moisture hardening one-component lacquers.
2. Description of the Prior Art
Moisture hardening polyurethane lacquers based on aliphatic polyisocyanates, in particular on isocyanate prepolymers containing aliphatically bound isocyanate groups are already known and have been described, for example, by H. Kittel in "Lehrbuch der Lacke und Beschichtungen", publishers W. A. Colomb, Berlin-Oberschwandorf 1973, Volume 1, Part 2, page 574. Polyisocyanates with a biuret structure prepared from hexamethylene diisocyanate have gained a position of particular commercial importance for the preparation of such binders.
The previously known moisture hardening one-component PUR lacquers have various advantages over two-component PUR lacquers, e.g., simple, uncomplicated methods of working up, great hardness and rapid onset of drying by the formation of high molecular weight polyurethane ureas. However, the moisture hardening one-component lacquers of the state of the art based on aliphatic isocyanates also have certain disadvantages which manifest themselves in the lack of long-term stability of the lacquers. Pigmented lacquers based on the isocyanate prepolymers previously known, for example, frequently do not satisfy the requirements for resistance to yellowing on exposure to light and for resistance to weathering. The insufficient resistance to weathering leads to loss of gloss and to chalking of the coatings produced from the lacquers.
It was an object of the present invention to provide new prepolymers containing isocyanate groups as binders for moisture hardening one-component lacquers which would be superior to the known prepolymers in the properties mentioned above.
This problem was solved by means of the process according to the invention described below.